A phase-locked-loop (PLL) is an electronic circuit having a voltage or current driven oscillator that is continuously adjusted to match in phase, and thus lock onto, the frequency of a reference signal. PLLs are frequently used in wireless communication systems, particularly where information is transmitted using frequency modulation (FM) (i.e. a method of impressing data onto an alternating-current wave by varying the instantaneous frequency of the wave).
A PLL generally consists of a voltage-controlled oscillator (VCO) that is tuned, for example, by means of a capacitive and inductive tank circuit. A phase comparator, charge pump, and low-pass-filter feedback loop enables the VCO to seek and lock onto a desired frequency based on a reference frequency. If the VCO frequency departs from the selected reference frequency, the phase comparator, in conjunction with the charge pump and low-pass-filter produces an error voltage that is applied to the VCO's tuning system to bring the VCO back to the reference frequency. The PLL, VCO, reference signal, charge pump, low-pass-filter, and phase comparator together with programmable divider in the feedback loop comprise what is commonly referred to as a frequency synthesizer.
In the case of a typical single-port VCO, the center frequency (fc) of the VCO output is modulated by applying varying input data to the programmable divider. The divider output and a reference signal having a frequency (FREF) is then applied to a phase detector that compares the phase of the reference signal with the phase of the divider output and applies a phase error signal to a loop low pass filter after processing by the charge pump. The voltage or current output of the filter is applied to the single VCO input to tune the VCO to the desired frequency.
Unfortunately, the single port modulation approach may not be suitable for high speed data applications; e.g. when the input data rate exceeds the PLL's natural loop frequency. For example, if the digital input data is changing at a rate of 1 megabit per second, and the loop response is, for example, 40 kHz, the loop response will filter out the changing data, and the modulation characteristics will be altered. Such is the case in a high data-rate frequency-shift keying (FSK) system. FSK is a method of transmitting digital signals having two binary states; i.e. a logic 0 (low) and a logic 1 (high), each of which is represented by a variance in frequency from fc. That is, a logic 0 is represented by a wave-form at a first specific frequency, and a logic 1 is represented by a wave-form at a different frequency.
A dual-port VCO includes a second modulation port for receiving an analog version of the input data thus combining a low pass response and a high pass response. This type of dual-port modulation permits modulation at frequencies well above the natural loop frequency. Thus, a certain degree of flexibility is achieved with regard to establishing the bandwidth of the main port loop without sacrificing input data rate. The architectures of single-port and dual-port synthesizers are well known, and further discussion is not deemed necessary. However, the interested reader is referred to U.S. Pat. No. 5,557,244 issued Sep. 11, 1996 and entitled “Dual-Port Phase and Magnitude Balance Synthesizer and Modulator and Method for Transceiver” and U.S. Pat. No. 5,912,926 issued Jun. 15, 1999 and entitled “Method of an Apparatus for Controlling Modulation of Digital Signals in Frequency Modulated Transmissions”.
FSK FM transmitters are designed to operate over a specific frequency band comprised of a plurality (e.g. 80) of different frequency channels each having a center frequency and upper and lower frequency limits. This specific frequency band is within the tuning range of a dual-port VCO of the type described above, and a dual-port synthesizer may be utilized to tune the VCO's output to desired channels. The VCO will then modulate the output carrier around the center frequency of the specific channel to which it is tuned.
It is desirable that the deviation of frequency about the center frequency of each channel be substantially constant over the specific frequency band. To accomplish this, the VCO's second port gain should remain constant over the operating range of the VCO. In practice, however, the gain associated with the second port (measured in frequency per volt; i.e. MHz/volt) is not constant but is in fact center frequency sensitive. That is, the second port VCO gain varies as the VCO is tuned over its operating band. This variation in the second port gain characteristic results in different changes in the instantaneous carrier frequency for substantially similar modulation voltages for different channels. For example, a desired deviation from the center frequency of each channel may be 170 kHz. Applying a specific modulation voltage to the VCO's second port to achieve this deviation may result in 120 kHz deviation in one channel and a 250 kHz deviation in another channel.
Several approaches have been utilized to avoid the problems associated with a varying second port gain characteristic. An entirely different architecture may be employed. That is, a single-port architecture may be utilized thus eliminating the problems associated with second port gain characteristics. Unfortunately, the benefits of high data-rate and low power consumption associated with dual-port VCOs are also eliminated. Alternatively, mechanical or electronic tuning devices may be employed. However, such devices would have to be incorporated into each unit significantly adding to production costs. Still another approach involves tightening process parameters during the manufacture of the VCO to reduce the VCO's second port gain variability across its frequency band. This likewise increases production costs and renders manufacturing significantly more complex. It should also be obvious that if the VCO is operated over a very narrow frequency band, the problems associated with variable second port gain characteristics are not significant. Unfortunately, certain applications require operation over a wide frequency band. For example, to satisfy the BLUETOOTH standard, the VCO must operate over an 80 MHz bandwidth.
It should therefore be appreciated that it would be desirable to provide a method and apparatus for compensating for deviation variances over the operating band of an FSK FM transmitter utilizing a dual-port VCO.